Extrinsic semiconductors rely on dopants to provide a desired density of charge carriers. Dopant implantation is a major step in extrinsic semiconductor processing. In conventional CMOS manufacturing, an ion beam implants dopants into the wafer. Chemical species are deposited into semiconductor wafers by bombarding the substrate with energized ions. The amount of dopant deposited controls the type and conductivity of the resulting semiconductor. The ability to control the active device characteristics depends on the ability to deposit a predetermined dosage of a dopant uniformly throughout the various regions of the substrate.
Commercially available ion implanter systems are used to perform implantation. The implanter includes an ion source head, which generates the energized ions. It has been observed experimentally that the source head lifetime was shorter when used to implant Ge than other species, such as As, B, BF2, and P. More severe tool symptoms were observed during Germanium (Ge) implantation, resulting in worse source head condition and poor beam uniformity and stability. The inventors observed a high current implanter experience poor source head life time during Ge implantation—as short as 10 hours mean time between failure. It was observed that WF6 whiskers were deposited on the aperture of the arc chamber of the ion source, degrading beam uniformity. The whiskers were the result of the tungsten material of the arc chamber wall being consumed in the following reaction:GeF4 gas+Ar(Dilution)+W(arc chamber)Ge++WF6+Ar++e−
The beam uniformity (measured as a ratio of the standard deviation of the beam current divided by the maximum beam current value) across the length of 300 mm was observed to be about 0.84%.
U.S. Patent Application Publication No. US 2008/0179545 A1, published Jul. 31, 2008 and entitled “Technique for Improving the Performance and Extending the Lifetime of an Ion Source with Gas Dilution” (“Perel et al.) is incorporated by reference herein in its entirety. Perel et al. describes an ion implanter system, an ion source therefor and a method for operating the ion source.
Perel et al. propose diluting the dopant gas with a dilutant gas containing 70% xenon (Xe) and 30% hydrogen (H2). The dilutant gas made up 10% to 40% of the total gas in the arc chamber with a preferred composition of 20% dilutant and 80% dopant gas. Perel et al. report reduced weight gain of ion source components with this combination.
Improved methods are desired.